tests: Increase Weekly Docker Container mem limit to 24GB

The following weekly tests were failing:
https://jenkins.gem5.org/job/weekly/53

This appears to be due to the "lulesh" tests failing with the docker
container running out of memory. This can be recreated with:

```
git clone -b develop https://gem5.googlesource.com/public/gem5

cd gem5

git clone -b develop https://gem5.googlesource.com/public/gem5-resources

docker pull gcr.io/gem5-test/gcn-gpu:latest
docker build -t hacc-test-weekly gem5-resources/src/gpu/halo-finder

docker run --rm -u $UID:$GID --volume $(pwd):$(pwd) -w \
    $(pwd) --memory="18g" hacc-test-weekly bash -c \
    "scons build/GCN3_X86/gem5.opt -j`nproc`"

docker run --rm --volume $(pwd):$(pwd) -w \
       "$(pwd)/gem5-resources/src/gpu/lulesh" \
       -u $UID:$GID --memory="18g" hacc-test-weekly make

docker run --rm -u $UID:$GID --volume "$(pwd)":"$(pwd)" -w \
    "$(pwd)" --memory="18g" \
    hacc-test-weekly build/GCN3_X86/gem5.opt configs/example/apu_se.py -n3 \
    --mem-size=8GB --reg-alloc-policy=dynamic \
    --benchmark-root="$(pwd)/gem5-resources/src/gpu/lulesh/bin" -c lulesh
```

It is currently unknown as to why this process consumes so much memory
or how it can be reduced, but increasing the docker container limit to
24GB for the Weekly tests does appear to fix the issue.

Change-Id: Ifcd65d4ab8bf6a12ddf7a45d61c779e691619072
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/60009
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/weekly.sh

@@ -33,7 +33,7 @@ dir="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
 gem5_root="${dir}/.."
 
 # The per-container Docker memory limit.
-docker_mem_limit="18g"
+docker_mem_limit="24g"
 
 # We assume the first two arguments are the number of threads followed by the
 # GPU ISA to test. These default to 1 and GCN3_X86 is no argument is given.